Selectively switchable wideband rf summer

ABSTRACT

A radio frequency (RF) summer circuit having a characteristic impedance Zo comprises first and second ports coupled by first and second resistances, respectively, to a junction. The circuit further comprises a series combination of a third resistance and a switch movable between open and closed positions and an amplifier having input and output terminals and operable in an off state and an on state wherein the series combination is coupled across the input and output terminals of the amplifier between the junction and a third port. The first resistance, second resistance, and the third resistance are all substantially equal to Z 0 /3. Further, when the switch is moved to the closed position and the amplifier is switched to the off state a passive mode of operation is implemented and when the switch is moved to the open position and the amplifier is switched to the on state an active mode of operation is implemented. The RF summer circuit develops a summed signal at the third port equal to a sum of signals at the first and second ports modified by one of first and second gain values.

FIELD OF DISCLOSURE

The present subject matter relates to radio frequency (RF) devices, andmore particularly, to a selectively switchable RF summer.

BACKGROUND

At times, it is necessary to control gain in an RF circuit. For example,an H-tree network may be used as an antenna feed for a phased-array RFdevice or an RF transversal filter. Signals in an H-tree network caneither sum coherently, creating a large signal at an associated antennafeed output port that can result in the production of non-lineardistortion at a device (e.g., an input amplifier) coupled to the outputport, or can sum incoherently, resulting in a reduction insignal-to-noise ratio (SNR) at another associated output port, albeit atreduced gain. A circuit designer can provide one or more signalsdeveloped at a corresponding number of H-tree network output port(s) toa downstream device(s). It is often necessary to balance noise andlinearity performance selectively at such port(s); however, it isdifficult to obtain gain control without sacrificing matching andbandwidth.

A prior approach to solving the foregoing issue may comprise a resistivecombiner with switched resistances, or a common variable gain amplifiertopology. These prior approaches, however, are only capable of achievinga relatively low dynamic range improvement in an H-tree summer.

SUMMARY

According to one aspect, a radio frequency (RF) summer circuit having acharacteristic impedance Z₀ comprises first and second ports coupled byfirst and second resistances, respectively, to a junction. The circuitfurther comprises a series combination of a third resistance and aswitch movable between open and closed positions and an amplifier havinginput and output terminals and operable in an off state and an on statewherein the series combination is coupled across the input and outputterminals of the amplifier between the junction and a third port. Thefirst resistance, second resistance, and the third resistance are allsubstantially equal to Z₀/3. Further, when the switch is moved to theclosed position and the amplifier is switched to the off state a passivemode of operation is implemented and when the switch is moved to theopen position and the amplifier is switched to the on state an activemode of operation is implemented. The RF summer circuit develops asummed signal at the third port equal to a sum of signals at the firstand second ports modified by one of first and second gain values.

Other aspects and advantages will become apparent upon consideration ofthe following detailed description and the attached drawings whereinlike numerals designate like structures throughout the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified circuit diagram of an embodiment of a selectivelyswitchable RF summer;

FIG. 2 is a simplified circuit diagram of the embodiment of aselectively switchable RF summer of FIG. 1 in a passive state ofoperation;

FIG. 3 is a simplified circuit diagram of the embodiment of aselectively switchable RF summer of FIG. 1 in an active state ofoperation;

FIGS. 4 and 5 are schematic diagrams of two realizations of theamplifier of FIG. 1;

FIGS. 6 and 9 are simplified schematic diagrams of the embodiment ofFIG. 1 in passive and active modes of operation, respectively, showingparasitic impedances of the amplifier;

FIGS. 7 and 8 are graphs illustrating S-parameters as a function offrequency for the passive mode of operation of FIG. 6; and

FIGS. 10 and 11 are graphs illustrating S-parameters as a function offrequency for the active mode of operation of FIG. 9.

DETAILED DESCRIPTION

Referring first to FIG. 1, a selectively switchable wideband RF summercircuit 20 comprises a multi-port device comprising first, second, andthird ports P1, P2, and P3, respectively. The first and second ports P1and P2 are coupled by resistors R1 and R2, respectively, to a junction22. A buffer amplifier U1 includes an input terminal coupled to thejunction 22 and an output terminal coupled to the third port P3. Theamplifier U1 includes power terminals coupled by first and secondswitches S1 and S2 to a supply voltage V+ and ground, respectively. Aseries combination of a third switch S3 and a third resistor R3 acrossthe amplifier U1 between the junction 22 and the third port P3.

Each of the first, second, and third switches S1, S2, and S3 isselectively operable between open and closed positions and may comprisea manually and/or mechanically operable device, an electronicallyoperable device (e.g., a transistor), or any other suitable device.Further, each of the first, second, and third resistors has a resistancerelated to a characteristic impedance Z₀ of the summer circuit 20.Specifically, in a preferred embodiment, the resistances of theresistors R1, R2, and R3 are the same and each is equal to Z₀/3. Thus,in a specific embodiment in which Z₀ is equal to 50 ohms, theresistances of R1, R2, and R3 are all equal to 50/3 (i.e., approximately16.67) ohms.

FIG. 2 illustrates the summer circuit 20 operable in a passive mode ofoperation. Such mode of operation is implemented by opening one or bothof the first and second switches S1 and S2 and closing the third switchS3. First and second input signals provided to the first and secondports P1 and P2, respectively, are summed at the junction 22 and thesummed signals are provided by the third switch S3 and the resistor R3to the third port P3. The amplifier U1 presents a very high impedance atthe input and output terminals (e.g., three times or greater withrespect to the impedance Z₀) inasmuch as the amplifier U1 is off at suchtime. The circuit 20 is impedance matched at all three ports P1, P2, andP3. and exhibits an approximate −6 db signal transmission gain betweenall three ports P1, P2, and P3. Further, inasmuch as the components inthe transmission path are substantially purely resistive, particularlywhen the switch S3 is manually or mechanically operable (thuseliminating parasitic impedance(s) of a transistor) operation in thepassive mode is substantially linear over a very wide range offrequencies (e.g., 0-40 Ghz.). The S-parameters for the circuit 20operating in the passive mode are illustrated in FIGS. 7 and 8. FIG. 6illustrates the circuit 20 of FIG. 2 with parasitic impedances of thebuffer amplifier U1 represented by capacitor C1 and resistor R4. TheS-parameters of FIGS. 7 and 8 are illustrated under the example that thecharacteristic impedance Z₀ is 50 ohms, the parasitic capacitor C1 has acapacitance of 20 femtofarads, and the parasitic resistance has aresistance of 500 ohms. Of course, as should be evident to one ofordinary skill in the art, the nature and values of parasitic impedancesvary with the choice of components, particularly the buffer amplifierU1, and the scope of the claims appended hereto is not limited to thenature and value(s) of the parasitic impedances or to the componentvalues as disclosed herein.

As seen in FIG. 7, the input port voltage reflection coefficients S₁₁and S₂₂ and the output port voltage reflection coefficient S₃₃ rise withfrequency between zero and 40 Ghz. because of the parasitic impedances.As seen in FIG. 8, the forward voltage gains S₃₁ and the parameters S₂₁and S₁₂ remain constant at about −6 db as noted previously.

Referring next to FIG. 3, the active mode of operation is initiated byclosing the switches S1 and S2 and opening the switch S3. The amplifierU1 is thus energized (i.e., turned on) and the resistor R3 is removedfrom the circuit. As in the passive mode of operation, first and secondinput signals provided to the first and second ports P1 and P2,respectively, are summed at the junction 22. In the active mode ofoperation, the summed signals are amplified by the amplifier U1 andprovided to the third port P3, with output impedance matching beingundertaken by the amplifier U1. As seen in FIG. 9, exemplary parasiticimpedances of the amplifier U1 comprising capacitor C1 and resistor R4are shown, which may have the same or different values as the parasiticimpedances shown in FIG. 6. The ports P1 and P2 remain partially matchedto Z₀ due to each input signal at the ports P1 and P2 seeing theopposite port's impedance.

FIGS. 10 and 11 illustrate S-parameters during operation of the circuit20 in the active mode. The input port voltage reflection coefficientsS₁₁ and S₂₂ remain at approximately equal levels over the zero to 40Ghz. bandwidth, as does the output port voltage reflection coefficient,albeit at a reduced level compared to the input port voltage reflectioncoefficients S₁₁ and S₂₂. The S-parameter S₃₁ remains substantiallyconstant at an approximate level of 8 db over the bandwidth (althoughnot shown in FIG. 11, the parameter S₃₂ is identical to the parameterS₃₁). The S-parameters S₂₁ and S₁₂ remain constant at about −3 db overthe bandwidth.

FIGS. 4 and 5 illustrate particular realizations of the amplifier U1 andassociated switches S1 and S2, it being understood that otherrealization could be used in place thereof. The exemplary embodiment ofFIG. 4 comprises P-channel and N-channel MOSFET's Q1 and Q2 havingsource and drain terminals connected in series between the switch S1 andthe switch S2 and interconnected gate terminals. A resistor R5 isconnected between the amplifier input terminal at the interconnectedgate terminals and the output terminal at a junction between the drainsof the transistors Q1 and Q2 to provide bias for proper operation.

The exemplary embodiment of FIG. 5 includes an N-channel MOSFET Q3 and aresistor R7 coupled to the drain terminal of the transistor Q3 at theamplifier output terminal. The series combination of the transistor Q3and the resistor R7 is coupled to the switch S1. The source terminal ofthe transistor Q3 is optionally coupled by the switch S2 to ground. Theswitch S2 acts as a redundant method of turning off the amplifier U1.The small parasitic series resistance that the switch S2 exhibits whenclosed can be desirable to control amplifier gain and linearity. If theswitch S2 is omitted the source terminal of the transistor Q3 is coupleddirectly to ground, thereby eliminating the small parasitic impedance. Agate terminal of the transistor Q3 is coupled to a junction between aseries-connected combination of a capacitor C2 and the resistor R6. Theseries connected combination of the capacitor C2 and the resistor R6 iscoupled between the amplifier input and a voltage V_(bias). Thecapacitor C2 and resistor R6 provide the proper fixed DC gate voltage tobias the transistor Q3.

INDUSTRIAL APPLICABILITY

The embodiments disclosed herein can be used as a widebandgain-controlling feature in H-tree RF signal summing networks. Specificapplications involve antenna feeds for phased-arrays and RF transversalfilters. Signals in H-tree networks will either sum coherently, creatinga large signal at the summed output port which can result in non-lineardistortion for the amplifier at the summed output, or will sumincoherently causing a reduction in signal-to-noise ratio (SNR) at theoutput. Use of the switchable active-passive 2-way RF signal summerdisclosed herein at each H-tree or other antenna feed output allows acircuit designer to select the passive mode of operation at ports wheresignals sum coherently or the active mode of operation at ports wheresignals sum incoherently. Thus, each signal level can be controlledindividually at different summing tiers to balance noise and linearityperformance.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

The use of the terms “a” and “an” and “the” and similar references inthe context of describing the invention (especially in the context ofthe following claims) are to be construed to cover both the singular andthe plural, unless otherwise indicated herein or clearly contradicted bycontext. Recitation of ranges of values herein are merely intended toserve as a shorthand method of referring individually to each separatevalue falling within the range, unless otherwise indicated herein, andeach separate value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context. The use of any and allexamples, or exemplary language (e.g., “such as”) provided herein, isintended merely to better illuminate the disclosure and does not pose alimitation on the scope of the disclosure unless otherwise claimed. Nolanguage in the specification should be construed as indicating anynon-claimed element as essential to the practice of the disclosure.

Numerous modifications to the present disclosure will be apparent tothose skilled in the art in view of the foregoing description. It shouldbe understood that the illustrated embodiments are exemplary only, andshould not be taken as limiting the scope of the disclosure.

We claim:
 1. A radio frequency (RF) summer circuit having acharacteristic impedance Z₀, comprising: first and second ports coupledby first and second resistances, respectively, to a junction; a seriescombination of a third resistance and a switch movable between open andclosed positions; and an amplifier having input and output terminals andoperable in an off state and an on state wherein the series combinationis coupled across the input and output terminals of the amplifierbetween the junction and a third port; wherein the first resistance,second resistance, and the third resistance are all substantially equalto Z₀/3; and wherein when the switch is moved to the closed position andthe amplifier is switched to the off state a passive mode of operationis implemented and when the switch is moved to the open position and theamplifier is switched to the on state an active mode of operation isimplemented and wherein the RF summer circuit develops a summed signalat the third port equal to a sum of signals at the first and secondports modified by one of first and second gain values.
 2. The RF summercircuit of claim 1, wherein the summer circuit is impedance matched atall ports and has substantially equal gains between all ports when thepassive mode of operation is implemented.
 3. The RF summer circuit ofclaim 1, wherein a magnitude of the sum of signals is modified by thefirst gain value when the passive mode of operation is implemented. 4.The RF summer circuit of claim 1, wherein a magnitude of the sum ofsignals is modified by the second gain value when the active mode ofoperation is implemented.
 5. The RF summer circuit of claim 1, whereinthe first gain value comprises a negative value and a magnitude of thesum of signals is modified by the first gain value when the passive modeof operation is implemented and the second gain value comprises apositive value and a magnitude of the sum of signals is modified by thesecond gain value when the active mode of operation is implemented. 6.The RF summer circuit of claim 1, wherein the amplifier is switchedbetween the off state and the on state by at least one further switch.7. The RF summer circuit of claim 6, wherein the at least one furtherswitch comprises two further switches and the amplifier comprises firstand second MOSFET's having source and drain terminals coupled betweenthe two further switches and gate terminals interconnected at the inputterminal of the amplifier wherein a junction between the first andsecond MOSFET's comprises the output terminal of the amplifier and aresistor is coupled between the input terminal and output terminal. 8.The RF summer circuit of claim 6, wherein the amplifier comprises aMOSFET having a drain terminal coupled to the output terminal of theamplifier, a first resistor coupled between the at least one furtherswitch and the output terminal of the amplifier and wherein acombination of a capacitor and second resistor is coupled between theinput terminal of the amplifier and a voltage and a junction between thecapacitor and the second resistor is coupled to a gate terminal of theMOSFET.
 9. The RF summer circuit of claim 8, wherein a still furtherswitch is coupled between a source terminal of the MOSFET and groundpotential.